Welch Allyn TM 262 Auto Tymp - User Manual - Manual (Page 77)

Revised 4/11/08 5-11
RS-232 Interface
Data Transmission Protocol
The RS-232 Interface uses a Auto Repeat Request (ARQ) communications protocol to ensure
the reliable transfer of data. With this protocol. the instrument will transmit a data record and
then wait for a response from the external device. If the external device receives the record cor-
rectly, is should respond with an acknowledge “ACK” character (ASCII control character
ACK). If the record is not received correctly, the external device should respond with a Not
AcKnowledge “NAK” character (ASCII control character NAK.)
If an ACK is received within 3 seconds of the completion of the transmission, the transmission
has been completed successfully. The “DATA TRANSFER” message is erased and normal
operation resumes.
If after the transmission of a record there is no response within three seconds, the record is
retransmitted with the same Record Sequence Number. If such a timeout occurs after the
second attempt, then the message “NO RESPONSE” is displayed, any pending transmissions
are aborted, and normal operation resumes
If a NAK response is received during a transmission or within three seconds after the transmis-
sion is completed, the record is retransmitted with the same Record Sequence Number. If the
transmission is not acknowledged within three attempts the message ‘TRANSFER NOT COM-
PLETE” is displayed for about three seconds, any pending transmissions are aborted, and
normal operation resumes.
The NAK responses and timeouts are treated independently. Thus, when a combination of
errors is happening, the NAKs and timeouts are being counted separately. If the transfer is not
successful, the error message displayed corresponds to the failure condition that occurs first.
The only expected response from the external device to your instrument is an ACK or a NAK
character. When a series of records are transmitted, the external device must ACK or NAK after
each record.